X-ray detectors designed as solid state detectors are known in X-ray imaging for the purpose of taking digital X-ray images of an examination object. In this case, an x-radiation is converted with the aid of a converter layer into electric charge and subsequently read out electronically by means of a downstream active matrix that includes a multiplicity of individual pixel readout elements arranged in rows. Subsequently, the imaging data representing the examination results are further processed for the image preparation, for example by correcting dark currents (noise) or sensitivity differences from the raw image data, that is to say the raw image data of the respective raw X-ray image.
It is known to carry out line noise correction (LNC) in order to correct the dark currents. To this end, at least one dark image value is picked up for each line and subtracted electronically from the raw image values of the respective line during the image preparation from pixel readout elements in edge regions of the active pixel matrix which are not fronted by a converter layer and which, in addition, are shielded with lead (Dark Reference Zone=DRZ).
It is known, furthermore, to prepare an offset image from dark image values for a correction of the dark currents, that is to say to take a picture without x-radiation and without the examination object, and to subtract it electronically from the raw image values after acquisition of the latter. The frequency with which the dark image values are picked up or offset images are prepared can vary; thus, a new offset image can be prepared, or a time interval can be selected for each X-ray picture.
Known active detector matrices have a detector plate made from amorphous silicon, for example a so called a-Si plate. In order to produce large area solid state detectors, a number of detector plates, for example, two, four or eight a-Si plates, are bonded to one another by way of a butting process. Even after a correction of the dark current on the X-ray image, striking brightness differences between different detector plates are often detected in the case of such compound active detector matrices. One cause of these so called half panel visibilities result from the use of detector plates made from different production runs. Even tiny differences during production can effect a large difference in the dark current.